A full focus imaging optimization method applied to multi-ply composites

Abstract

The present application relates to the technical field of nondestructive testing, in particular to a full focus imaging optimization method applied to multi-ply composite materials. The present application optimizes the full focus imaging of multi-ply composite materials through a systematic process: first, FMC data of a defect-free area is collected, combinations of elastic constants of each ply are enumerated and sound velocity curves are calculated, then full focus images are generated based on equivalent sound velocity, and finally the optimal elastic constants are screened according to the average amplitude of the bottom surface. The elastic constant combination covers a large angle of sound velocity information, solving the problem of limited angle coverage, and has higher imaging accuracy for large thickness composite materials; the image amplitude is used to automatically screen parameters, avoiding the inefficiency and error of manual identification of peak values of attenuation signals, and at the same time improving the upper bound of the range of detectable propagation angles; the sound velocity is calculated in combination with the characteristics of non-homogeneous plies, so that the defect recognition accuracy is greatly improved compared with the traditional homogeneous model.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nondestructive testing, and particularly relates to a full-focus imaging optimization method applied to multi-ply composite materials. BACKGROUND

[0002] When defects of a composite material are identified, a bottom echo method is usually used to calibrate the sound velocity, and then subsequent defect identification steps are performed based on the calibrated sound velocity. Although the bottom echo method is widely used, it has the following disadvantages:

[0003] 1. Limited angle coverage: The effective signal propagation angle range that can be collected is small due to the physical length of the ultrasonic array. However, defect identification requires sound velocity curve information with a larger propagation angle.

[0004] 2. Difficulty in peak extraction: For ultrasonic waves with a larger propagation angle, the propagation distance is longer, resulting in significant attenuation of the bottom echo signal and weak peaks, which often require human intervention for judgment, with low efficiency and poor reliability.

[0005] 3. Poor model applicability: The bottom echo method is based on the assumption of homogeneous materials, and the sound velocity curve obtained by bottom calibration is directly applied to all areas of imaging, without considering the sound velocity variation of different plies (non-homogeneous areas) of the composite material.

[0006] Under the combined action of the above disadvantages, the defect identification accuracy of multi-ply composite materials is severely restricted, and effective solutions are urgently needed. SUMMARY

[0007] In order to avoid and overcome the technical problems existing in the prior art, the present application provides a full-focus imaging optimization method applied to multi-ply composite materials. The present application effectively improves the defect identification accuracy of multi-ply composite materials through adaptive correction of the sound velocity curve of the composite material at different depths.

[0008] To achieve the above object, the present application provides the following technical scheme:

[0009] A full-focus imaging optimization method applied to multi-ply composite materials, comprising the following optimization steps:

[0010] S1, obtaining FMC data of the multi-ply composite material in a defect-free area;

[0011] S2, determining all values of the elastic constants of each ply, and combining all values of the elastic constants related to longitudinal waves of different plies with each other to obtain all related elastic constant combinations of the multi-ply composite material;

[0012] S3, calculating the original sound velocity curve of the ultrasonic wave under each set of elastic constant combination;

[0013] S4, obtaining an equivalent sound velocity curve corresponding to each original sound velocity curve based on the ratio of distance to time;

[0014] S5, for each set of elastic constant combination, generating a corresponding full focus image according to the corresponding individual equivalent sound velocity curve and the FMC data in step S1;

[0015] S6, obtaining the average amplitude of the bottom surface area of each full focus image respectively, and selecting the elastic constant combination corresponding to the maximum average amplitude as the optimal elastic constant;

[0016] S7, obtaining the FMC data of the multi-layer composite material with a defect area, and executing steps S3-S5 using the FMC data and the optimal elastic constant to generate a full focus image with a defect area.

[0017] As a further scheme of the application: the calculation process of the original sound velocity curve is as follows:

[0018] S31, selecting an optional layer of the multi-layer composite material as a base layer, and establishing a three-dimensional reference coordinate system on the base layer, and the X axis of the reference coordinate system and the fiber direction of the layer are parallel to each other; for each set of elastic constant combination, executing steps S32-S39;

[0019] S32, under the current elastic constant combination, establishing the physical relationship between the phase velocity of ultrasonic wave in the base layer and the elastic constant through Christoffel equation, and constructing the stiffness matrix composed of each elastic constant combination as follows:

[0020] S33, calculating the phase velocity of ultrasonic wave in the base layer based on the stiffness matrix;

[0021] S34, calculating the group velocity of ultrasonic wave in the base layer based on the phase velocity;

[0022] S35, traversing the propagation angle range, calculating the group velocity corresponding to each propagation angle; and taking the propagation angle as the horizontal coordinate and the group velocity as the vertical coordinate, sequentially smoothing connecting each group velocity to obtain the original sound velocity curve of the base layer;

[0023] S36, constructing the coordinate rotation matrix of the rotation layer which rotates relative to the base layer;

[0024] S37, calculating the rotation stiffness matrix of the current rotation layer based on the coordinate rotation matrix;

[0025] S38, substituting the rotation stiffness matrix of the current rotation layer into step S33, and executing steps S33-S35 to obtain the original sound velocity curve of the current rotation layer;

[0026] S39, step S38 is performed on each layer of the multi-layer composite material to obtain the original sound velocity curve of each rotation layer in the multi-layer composite material.

[0027] As a further scheme of the present application: the stiffness matrix is expressed as follows:

[0028] (1);

[0029] wherein, represents the stiffness matrix of the base layer; represents the tensile stiffness along the positive direction of the X axis; represents the coupling stiffness in the XY plane; represents the coupling stiffness in the XZ plane; represents the tensile stiffness along the positive direction of the Y axis; represents the coupling stiffness in the YZ plane; represents the tensile stiffness along the positive direction of the Z axis; represents the shear stiffness around the X axis; represents the shear stiffness around the Y axis; represents the shear stiffness around the Z axis; under the transverse isotropy condition, , , , .

[0030] As a further scheme of the present application: the phase velocity is calculated according to the following formula:

[0031] (2);

[0032] (3);

[0033] (4);

[0034] (5);

[0035] (6);

[0036] wherein, and both represent transition parameters; represents the material density of the base layer; represents the sine value of the included angle between the ultrasonic wave phase velocity propagation direction and the positive direction of the Z axis in the base layer ; and represents the included angle between the ultrasonic propagation direction and the positive direction of the Z axis in the base layer The cosine value of . and By correlating the phase velocity angle with the sound velocity and solving the square root expression using transition parameters, the phase velocity is ensured to conform to the physical meaning of the wave equation. The phase velocity angle (0°-90°) is directly incorporated into the phase velocity calculation, resolving the large angle extrapolation errors found in traditional methods. The formula derivation is based on solving the eigenvalues ​​of the Christoffel equation, avoiding nonphysical solutions (such as imaginary velocity). This ensures that the phase velocity calculation is strictly consistent with the phase propagation characteristics of ultrasound in anisotropic materials, laying a precise foundation for group velocity calculations.

[0037] As a further solution of the present invention: group velocity The calculation formula is as follows:

[0038] (7);

[0039] (8);

[0040] (9);

[0041] ;

[0042] Where, Represents the group velocity component of the ultrasonic wave along the positive direction of the X axis; represents the group velocity component of the ultrasonic wave along the positive direction of the Z axis, Represents the ultrasonic propagation angle. Decompose the group velocity into X-axis components and the Z-axis component The actual propagation velocity is synthesized by square root. The direction of the group velocity is consistent with the propagation direction of ultrasonic energy and matches the stress distribution between composite layers. It can more accurately describe the difference in the propagation of acoustic energy in the fiber direction and the perpendicular direction, providing a more realistic velocity parameter for sound velocity curve fitting.

[0043] As a further solution of the present invention: coordinate rotation matrix It is expressed as follows:

[0044] (10);

[0045] in, Indicates the rotation angle of the current rotating layer relative to the reference coordinate system around the Z axis. Indicates the rotation angle of the ply around the Z axis. 、 The same directly corresponds to the shear effect after the rotation of the coordinate system. The coordinate transformation of any lay-up angle (such as ±45°, 90°) of 0°-180° is supported, covering the common lay-up structure of composite materials; the matrix form supports GPU parallel computing, and the calculation time of the rotational stiffness matrix is shortened when processing multi-layer composite materials; the geometric transformation process is intuitive and interpretable, which is convenient for engineers to understand the influence mechanism of the lay-up angle on the sound velocity calculation, and improves the model debugging efficiency.

[0046] As a further scheme of the present application: the calculation formula of the rotational stiffness matrix is as follows:

[0047] (11) ;

[0048] In the formula, denotes the rotational stiffness matrix, denotes the transpose matrix of . The stiffness matrix of the basic lay-up is converted into the stiffness matrix of the rotational lay-up by using , wherein the transpose matrix ensures the energy conservation (the value of the matrix determinant is unchanged) before and after the transformation. The angle mapping of the anisotropic characteristics is realized through linear algebra operation, so that the stiffness matrix of the rotational lay-up strictly conforms to the change of the fiber direction; the physical parameter distortion (such as the non-positive definite problem of the stiffness matrix) caused by the coordinate transformation is avoided, and the physical rationality of the subsequent sound velocity calculation is ensured; the matrix transformation process can be realized automatically through programming, and is suitable for batch calculation demand of multi-lay-up complex structure.

[0049] As a further scheme of the present application: the acquisition process of the equivalent sound velocity curve is as follows:

[0050] S41, in the reference coordinate system, the propagation time of the ultrasonic wave from the ultrasonic probe element point to the focus point under the current propagation angle is calculated :

[0051] (12) ;

[0052] In the formula, denotes the number of layers in which the focus point is located in the multi-lay-up composite material; denotes the propagation distance of the ultrasonic wave in the first layer lay-up, denotes the group velocity of the ultrasonic wave in the first layer lay-up;

[0053] S42, based on the propagation time, the equivalent velocity is calculated by the ratio of distance to time;

[0054] (13) ;

[0055] wherein, represents the focal point of the ultrasonic wave in the layer; represents the focal point of the ultrasonic wave in the layer; represents the Euclidean distance between the array element point

[0056] S43, traverse the propagation angle range, calculate the corresponding equivalent speed of each layer in each propagation angle, and sequentially smooth connect each equivalent speed to obtain the equivalent sound speed curve of each layer, with the propagation angle as the horizontal coordinate and the equivalent speed as the vertical coordinate.

[0057] The equivalent speed is defined by the ratio of the Euclidean distance to the total propagation time, and the multi-layer broken line propagation is simplified to straight line distance calculation. The equivalent sound speed curve is smoothly fitted with "angle-speed", which can be directly substituted into the full focusing imaging formula to avoid the time-consuming operation of point-by-point sound speed correction in the traditional method; by accumulating the propagation time of each layer, the influence of the non-homogeneous characteristics of the material on the sound speed is considered comprehensively, so that the equivalent sound speed is closer to the average propagation speed of the ultrasonic wave in the actual path, and the imaging precision is improved.

[0058] As a further scheme of the present application: the generation process of the full focusing image in step S5 is as follows:

[0059] S51, select any array element in the ultrasonic transmitting probe as the transmitting array element, and the remaining array elements as the receiving array elements; calculate the total propagation time when the ultrasonic wave is emitted from the transmitting array element, propagates to the focal point , and then is reflected to the ultrasonic transmitting probe and is received by each receiving array element;

[0060] (14);

[0061] wherein, represents the total propagation time when the ultrasonic wave is emitted from the first array element, propagates to the focal point , and then is reflected to the ultrasonic transmitting probe and is received by the first receiving array element; represents the coordinate of the first array element in the reference coordinate system XZ plane as the transmitting array element; represents the coordinate of the first array element in the reference coordinate system XZ plane as the receiving array element;

[0062] S52, for each focal point, traverse all transmitting-receiving array element pairs , obtain each FMC data ; extract the signal amplitude in from the FMC data , and accumulate to obtain the focal point​ synthetic signal amplitude at each focus point ;

[0063] (15);

[0064] S53, obtain the synthetic signal amplitude at each focus point to form a full focus image by combination.

[0065] The total propagation time of all transmitting-receiving element pairs is calculated, and the signal amplitude of the corresponding FMC data is accumulated, the defect reflection signal is accumulated and enhanced at the imaging point by using the ultrasonic coherence, while the random noise of the non-defect area is suppressed; considering the spatial position relationship between the focus point and all elements, a two-dimensional focusing effect is formed, and the recognition rate of the internal defects of the composite material is greatly improved; the full matrix acquisition and accumulation algorithm is compatible with the existing ultrasonic array hardware, and the imaging quality upgrading can be realized without additional hardware cost.

[0066] As a further scheme of the present application: the process of obtaining the optimal elastic constant is as follows:

[0067] S61, for each full focus image, the region with a thickness H set from the bottom surface upward is selected as the bottom surface region;

[0068] S62, calculate the average synthetic signal amplitude in the bottom surface region in each full focus image ;

[0069] ;

[0070] In the formula, and respectively, the X-axis coordinate of the leftmost element of the phased array probe and the X-axis coordinate of the rightmost element; K is the number of pixel points contained between and at the depth of H.

[0071] S63, obtain the full focus image with the maximum average synthetic signal amplitude , and combine the elastic constant of the full focus image as the optimal elastic constant.

[0072] The average synthetic signal amplitude of the region with the set thickness of the bottom surface upward is calculated, and the maximum value is combined with the elastic constant to obtain the optimal solution. The bottom surface echo amplitude is strongly related to the calibration accuracy of the sound velocity, and the matching degree of the material parameters can be directly reflected by taking the amplitude as an index. The instability of extracting the signal peak point in the traditional method is solved by taking the bottom surface amplitude of the full focus image as an evaluation index, and the range of the detectable propagation angle is expanded, and more material acoustic characteristics are included. The ROI region is set to avoid the interference of surface defects or boundary effects, and the screening reliability is improved. Without presetting the material parameters, the data-driven automatic optimization is realized, the detection scene of the multi-layer composite material is adapted, and the screening process can be automatically realized through programming, the cost of manual intervention is reduced, and the detection efficiency is improved.

[0073] Compared with the prior art, the beneficial effects of the present application are:

[0074] 1. The full focus imaging of the multi-layer composite material is optimized through a systematic process: FMC data of a defect-free area is collected, combinations of elastic constants of each layer are enumerated and sound velocity curves are calculated, then full focus images are generated based on the equivalent sound velocity, and finally the optimal elastic constant is screened by the average amplitude of the bottom surface. The core advantage is to break through the limitations of the traditional bottom surface echo method, which covers a wide range of sound velocity information by combining elastic constants, solves the problem of limited angle coverage, automatically screens parameters by image amplitude, avoids the inefficiency and error of manual identification of signal peaks, calculates the sound velocity based on the characteristics of non-homogeneous layers, greatly improves the defect recognition accuracy compared with the traditional homogeneous model, and has high automation degree, which is suitable for composite material detection scenarios.

[0075] 2. The anisotropic characteristics of each layer of the composite material are integrated into the sound velocity calculation, and the recursive relationship between phase velocity and group velocity is used to make the sound velocity curve consistent with the actual propagation law of ultrasonic waves. The layered modeling strategy solves the influence of the rotation angle of the multi-layer on the sound velocity, provides sound velocity parameters matched with the material structure for subsequent imaging, and effectively reduces the proportion of imaging blind area.

[0076] 3. The complex material parameters are decoupled into standardized matrix elements, which is convenient for experimental measurement and simulation input. The independent elastic constants are simplified by assuming transverse isotropy, which reduces the calculation complexity and adapts to rapid engineering modeling. The matrix form is compatible with finite element software (such as ABAQUS), which can directly interface with existing simulation platforms, and improves the model landing efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0077] Figure 1 The flowchart of the present application.

[0078] Figure 2 The reference coordinate system in the present application.

[0079] Figure 3 The layer direction in the present application.

[0080] Figure 4 For Figure 3 The equivalent sound velocity curve corresponding to the different rotation angles of the layers in the present application.

[0081] Figure 5 The full-focus image containing defects in the present application. DETAILED DESCRIPTION

[0082] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0083] Please refer to Figures 1-4 In the embodiments of the present application, the ultrasonic measurement device for detection includes: a 64-element ultrasonic probe with a center distance of 0.60 mm and a nominal center frequency of 5 MHz, a full-matrix acquisition phased array controller, a computer with a phased array detection platform operating environment, and a composite material acoustic test block containing built-in defects. The established reference coordinate system is shown in Figure 2 The density value of the carbon fiber multi-layer plate is 1566 m 3 / kg, the thickness is 10 mm, and the layers are arranged in the manner shown in , a total of 120 layers, as shown in Figure 3 , the arrow in Figure 3 indicates the depth direction. A delamination defect with a diameter of 6 mm is simulated by a polytetrafluoroethylene film in the plate, which is buried at a distance of 8.5 mm from the surface. The reliability of the present application is verified by experimental delamination defect detection of the composite material by ultrasonic phased array detection technology.

[0084] I. Obtain FMC data of the defect-free area

[0085] When performing FMC (Full Matrix Capture) data acquisition in a defect-free area, the main concern is the process of data acquisition and the integrity of the data, rather than the identification or characterization of defects. FMC data acquisition is a technology for ultrasonic imaging that does not require prior knowledge (such as shape, sound velocity, etc.) of the object to be inspected (such as metal parts, composite materials, etc.). The following are the detailed steps and precautions for FMC data acquisition in a defect-free area:

[0086] 1. Preparation

[0087] Equipment preparation: Ensure that the phased array ultrasonic instrument and related software are operating normally, and that the probe is connected correctly.

[0088] Calibration: Perform necessary calibration of the instrument to ensure accuracy of data acquisition.

[0089] Setting Parameters: Set appropriate detection parameters such as frequency, gain, focal depth, etc., based on the material properties of the inspected component and detection requirements.

[0090] 2、FMC Data Acquisition Steps

[0091] 2.1、Excitation and Reception

[0092] Single-element excitation: First, select one element in the array as the transmitter to excite ultrasonic waves.

[0093] Full-array reception: While the transmitter is exciting ultrasonic waves, all other elements in the array act as receivers to receive ultrasonic echo signals from the inspected region.

[0094] Repeat the process: Repeat the above process by selecting a new element as the transmitter each time until all elements have been rotated through as transmitters. In this way, each element will have transmitted ultrasonic waves and all other elements will have received echo signals from those ultrasonic waves.

[0095] 2.2、Data Recording

[0096] Record A-scan waves: During each transmission and reception process, record the A-scan (time-amplitude waveform) data received by each element. This data contains information about the propagation and reflection of ultrasonic waves in the inspected region.

[0097] Store data: Store all transmitted and received data from all elements in the instrument or computer for subsequent TFM (Total Focusing Method) image reconstruction.

[0098] 3、Notes

[0099] Ensure completeness: During data acquisition, ensure that every element is rotated through as a transmitter and that all received data is recorded completely.

[0100] Avoid interference: During data acquisition, try to avoid external interference (such as electromagnetic interference, mechanical vibration, etc.) to ensure the accuracy of the data.

[0101] Quality control: Regularly maintain and check the instrument to ensure its stability and reliability during data acquisition.

[0102] 4、Subsequent Processing

[0103] TFM image reconstruction: After the FMC data acquisition is completed, the TFM algorithm can be used to process the collected data to generate high-resolution ultrasound images. The TFM algorithm enhances the signal intensity of the target imaging area by superimposing all received ultrasound signals, thereby improving the clarity and resolution of the image.

[0104] In summary, the main concern of FMC data acquisition in a defect-free area is the integrity and accuracy of the data acquisition. Through reasonable setting and strict operation process, it can be ensured that the collected data can meet the needs of subsequent TFM image reconstruction.

[0105] II. Determine the combination of elastic constants

[0106] From formulas (3)~(4), there are four elastic constants related to the quasi-longitudinal wave in anisotropic materials, respectively, , , , ; among them, The value can be determined in advance according to the pulse echo mode to simplify the entire optimization process. When the ultrasonic wave propagates along the direction perpendicular to the array, i.e. 0°, according to formulas (1)~(4), , , the phase velocity and the group velocity of the quasi-longitudinal wave degenerate to . The group velocity can be obtained by the time of flight from the transmitting element to the bottom surface of the pulse echo signal, combined with the thickness H of the carbon fiber composite material, the expression of is:

[0107] (16);

[0108] After formula (16) is calculated, the final value of

[0109] For the remaining parameters related to longitudinal wave imaging, this embodiment sets the range of to , the sampling interval is 5GPa, and there are 21 sampling points. The range of is , the sampling interval is 2GPa, and there are 6 sampling points. The range of is

[0110] , the sampling interval is 2GPa, and there are 10 sampling points.

[0110] III. Calculate the original sound velocity curve

[0111] Substitute these elastic constants into formulas (1)-(5) in turn to calculate the sound velocity curve under the base ply at 0°.

[0112] In combination with formulas (6)-(7), calculate the stiffness matrix under the ply at 90°, ±45° C , and calculate the original sound velocity curve under the ply at 90°, ±45°.

[0113] Four, calculate the equivalent sound velocity curve

[0114] According to formula (8), the equivalent sound velocity curve is calculated, and the calculation result is shown in Figure 4 .The equivalent sound velocity curve calculated according to formula (8) is shown in Figure 4 , and the "bottom surface" marked in the figure indicates that the sound velocity measurement is based on the bottom surface reflection signal of the material. The curve as a whole presents a certain fluctuation trend, and through analysis of the variation characteristics of the curve, basis can be provided for subsequent defect identification or material performance evaluation. Figure 4

[0115] Five, full focus image

[0116] According to formula (16), full focus imaging is carried out, the selected imaging area is the cross section below the array, the size is 37.8mm×12mm, the length of the grid point in the X axis direction is 0.1 times the interval between adjacent elements, the width in the Z axis direction is 0.5 times the thickness of a single ply, and the number of grids in the imaging range is 631×313.

[0117] Six, obtain optimal elastic constant

[0118] Since the thickness of the known material is 10mm, according to the mean value of the echo amplitude at the bottom surface 10mm of the full focus image as the evaluation index, there are 21×2×10=420 output points according to step 2. Find the sampling point corresponding to the maximum evaluation index, the result shows that the elastic constant vector related to the longitudinal wave , that is, the optimal elastic constant.

[0119] Seven, defect identification

[0120] Store the above optimal elastic constant corresponding equivalent sound velocity curve. Apply the equivalent sound velocity curve to the full focus imaging of the multi-ply composite material plate containing defects to realize the identification of the defects, and the imaging result is shown in Figure 5 . Figure 5The full focus imaging results based on the optimal elastic constant equivalent sound velocity curve are shown for identifying defects in the multi-layer composite plate. The horizontal axis (-15 to 15) represents the spatial position, the left vertical axis (0 to 10) reflects the depth of the composite material, and the right vertical axis (0 to 1) reflects the signal amplitude or energy intensity. Figure 5 The medium red and yellow area corresponds to the defect position, mainly concentrated near the horizontal axis 0, consistent with the peak area of the curve, indicating that there is obvious energy reflection or abnormality here. The full focus imaging technology realizes high-precision defect detection by synthesizing the sound wave path and combining the equivalent sound velocity curve optimization, and the red and yellow gradient area intuitively presents the distribution and severity of the defects.

[0121] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A full focus imaging optimization method applied to multi-ply composite materials, characterized in that: The optimization steps include: S1. Obtain FMC data of a multi-ply composite material with no defect areas; S2. determining all possible values ​​of the elastic constants of each ply, and combining all possible values ​​of the elastic constants of different plies related to longitudinal waves with one another to obtain all possible combinations of relevant elastic constants of the multi-ply composite material; S3, calculating the original sound velocity curve of the ultrasonic wave under each set of elastic constant combinations; S4. Based on the ratio of distance to time, obtain an equivalent sound speed curve corresponding to each original sound speed curve; S5. For each set of elastic constant combinations, a corresponding fully focused image is generated according to the corresponding equivalent sound velocity curves and the FMC data in step S1; S6. Obtain the average amplitude of the bottom area of ​​each fully focused image, and select the elastic constant combination corresponding to the maximum average amplitude as the optimal elastic constant; S7 . Obtain FMC data of the multi-ply composite material with the defective area, and execute steps S3 - S5 using the FMC data and the optimal elastic constants to generate a fully focused image with the defective area.

2. The total focus imaging optimization method for multi-ply composite materials according to claim 1, characterized in that: The calculation process of the original sound speed curve is as follows: S31. Selecting one ply in the multi-ply composite material as a base ply, and establishing a three-dimensional reference coordinate system on the base ply, with the X-axis of the reference coordinate system parallel to the fiber direction of the ply; executing steps S32-S39 for each set of elastic constant combinations; S32. Under the current elastic constant combination, the physical relationship between the phase velocity of the ultrasonic wave and the elastic constant in the foundation ply is established through the Christoffel equation, and a stiffness matrix composed of the various elastic constants is constructed; S33. Calculate the phase velocity of the ultrasonic wave in the foundation ply based on the stiffness matrix; S34. Calculate the group velocity of the ultrasonic wave in the base ply based on the phase velocity; S35, traversing the propagation angle range, calculating the group velocity corresponding to each propagation angle; and using the propagation angle as the horizontal coordinate and the group velocity as the vertical coordinate, smoothly connecting each group velocity in sequence to obtain the original sound velocity curve of the foundation ply; S36, constructing a coordinate rotation matrix of the rotating ply that rotates relative to the base ply; S37. Calculate the rotation stiffness matrix of the current rotating ply based on the coordinate rotation matrix; S38, substituting the rotational stiffness matrix of the current rotational ply into step S33, and executing steps S33-S35 to obtain the original sound velocity curve of the current rotational ply; S39. Execute step S38 for each ply in the multi-ply composite material to obtain an original sound velocity curve of each rotating ply in the multi-ply composite material.

3. The total focus imaging optimization method for multi-ply composite materials according to claim 2, characterized in that: The stiffness matrix is ​​expressed as follows: ； in, represents the stiffness matrix of the foundation ply; Indicates the tensile stiffness along the positive direction of the X axis; represents the coupling stiffness in the XY plane; represents the coupling stiffness in the XZ plane; Indicates the tensile stiffness along the positive direction of the Y axis; represents the coupling stiffness in the YZ plane; Indicates the tensile stiffness along the positive direction of the Z axis; represents the shear stiffness around the X-axis; represents the shear stiffness around the Y axis; represents the shear stiffness around the Z axis; under transversely isotropic conditions, , , , .

4. The total focus imaging optimization method for multi-ply composite materials according to claim 3, characterized in that: Phase velocity The calculation formula is as follows: ； ； ； ； ； Where, and Both represent transition parameters; Indicates the material density of the base ply; Indicates the angle between the propagation direction of the ultrasonic phase velocity in the base layer and the positive direction of the Z axis The sine value of Indicates the angle between the ultrasonic propagation direction in the base layer and the positive direction of the Z axis The cosine of .

5. The total focus imaging optimization method for multi-ply composite materials according to claim 4, characterized in that: Group velocity The calculation formula is as follows: ； ； ； ； Where, Represents the group velocity component of the ultrasonic wave along the positive direction of the X axis; represents the group velocity component of the ultrasonic wave along the positive direction of the Z axis, Indicates the ultrasonic wave propagation angle.

6. The total focus imaging optimization method for multi-ply composite materials according to claim 5, characterized in that: Coordinate rotation matrix It is expressed as follows: ； in, Indicates the rotation angle of the current rotated ply relative to the reference coordinate system around the Z axis.

7. The total focus imaging optimization method for multi-ply composite materials according to claim 6, characterized in that: The calculation formula of the rotational stiffness matrix is ​​as follows: ； Where, represents the rotational stiffness matrix, express The transposed matrix of .

8. The total focus imaging optimization method for multi-ply composite materials according to claim 7, characterized in that: The process of obtaining the equivalent sound speed curve is as follows: S41. In the reference coordinate system, calculate the current propagation angle of the ultrasonic wave from the ultrasonic probe array element point Spread to the focal point The propagation time : ； Where, Indicates the number of plies where the focal point is located in a multi-ply composite material; Indicates that ultrasound Propagation distance in a layer stack, Indicates that ultrasound Group velocity in a ply layup; S42. Calculate the equivalent speed based on the ratio of distance to time based on the propagation time; ； Where, Indicates focus The equivalent speed of ultrasonic wave propagation in the ply; Indicates focus Ultrasonic probe array element point The Euclidean distance between S43. Traverse the propagation angle range, calculate the equivalent velocity corresponding to each layer at each propagation angle, and use the propagation angle as the horizontal coordinate and the equivalent velocity as the vertical coordinate to smoothly connect each equivalent velocity in sequence to obtain the equivalent sound velocity curve of each layer.

9. The total focus imaging optimization method for multi-ply composite materials according to claim 8, characterized in that: The process of generating the all-focus image in step S5 is as follows: S51, select one array element from the ultrasonic transmitting probe as the transmitting array element, and the rest of the array elements are used as receiving array elements; calculate the ultrasonic wave emitted from the transmitting array element and propagated to the focal point The total propagation time after it is reflected into the ultrasonic transmitting probe and received by each receiving array element; ； Where, Indicates that the ultrasonic wave The array elements are emitted and spread to the focal point After that, it is reflected into the ultrasonic transmitting probe and The total propagation time when each receiving element receives; Indicates the first transmitting element The coordinates of each array element in the XZ plane of the reference coordinate system; Indicates the first receiving element The coordinates of each array element in the XZ plane of the reference coordinate system; S52: For each focal point, traverse all transmit-receive array element pairs , get each FMC data ;From FMC data Extract The signal amplitude in the , and accumulate to get the focus point The composite signal amplitude at ; ； S53: Obtain the synthetic signal amplitudes at all focus points, and combine them to form a fully focused image.

10. The total focus imaging optimization method for multi-ply composite materials according to claim 9, characterized in that: The process of obtaining the optimal elastic constants is as follows: S61, for each fully focused image, select an area of ​​the bottom surface with a thickness H upward as the bottom surface area; S62: Calculate the average synthetic signal amplitude within the bottom area of ​​each fully focused image. : ； Where, and are the X-axis coordinates of the leftmost element and the rightmost element of the phased array probe respectively; K At depth H, and The number of pixels contained in between; S63, obtaining the maximum average synthetic signal amplitude The full-focus image is obtained, and the elastic constant combination of the full-focus image is taken as the optimal elastic constant.

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